1. Field of the Invention
This invention relates to a process for making solid alkaline detergents at low temperatures and a method for preparing detergents containing nitrilotriacetate.
2. Description of the Prior Art
In conventional institutional and industrial spray washing machines, detergents are added to the wash tank by means of automatic dispenser systems. These detergents generally have high levels of alkalinity. Accordingly, they contain alkali metal hydroxides such as sodium hydroxide as well as chemicals which are particularly useful for hard surface cleaning. Examples of these have included phosphates, silicates, chlorine-containing compounds, defoamers and organic polyelectrolyte polymers and the like.
One major problem with automatic dish washing detergents is the inability of these detergents to be easily measured and dispensed. Solid or cast detergents provide a means whereby the safety, convenience and performance of the detergent and cleaning system can be enhanced. The use of solid cast detergents minimizes contact between the user and the high performance or high alkalinity detergent composition. Additionally, such cast detergents provide ease in insertion and replacement.
One problem found in both solid cast detergent compositions and powder detergent compositions is caused by the differing solubilities of the various components in water. Because the components of standard detergents dissolve at differing rates or have different equilibrium solubilities, the first effluent from a solid cast detergent may be overly rich in certain compounds while lacking other key detergent compounds causing the effectiveness of the detergent to vary greatly through the wash cycle or from washing to washing.
Another problem encountered in the manufacture and use of solid, cast detergents is the instability of certain detergent components at high temperatures or under highly alkaline conditions. Generally, in order to form a homogeneous, cast alkaline detergent, an aqueous solution of a caustic material such as sodium hydroxide is melted at temperatures above the "freeze" point for the given solution. For sodium hydroxide solutions of about 65 to about 75 percent, this temperature is above 144.degree. to 146.degree. F. Other desired detergent components can be added to the melted caustic solution and mixed to assure uniform dispersion. The melted solution can, then, be poured into the desired molds. As the caustic solution cools, it solidifies forming the solid cast detergent.
Various sequestrants have been incorporated in the caustic melts with varying degrees of success. One such sequestrant, sodium tripolyphosphate, is extremely effective but difficult to incorporate in solid cast detergent. Sodium tripolyphosphate exhibits instability at high temperatures in alkaline media and is known to revert to sodium tetrapyrophosphate and sodium orthophosphate under such conditions. These reversion products have less sequestering ability than that of sodium tripolyphosphate.
Methods of manufacturing cast detergents which overcome this reversion problem have been proposed. U.S. Pat. No. 4,569,780 to Fernholz et al outlines a method for making solid cast detergent in which an alkali metal hydroxide is heated to a temperature above its melt point. Alkaline hydratable compounds such as sodium tripolyphosphate present in an alkaline solution can then be added to the melt. In order to work, the sodium hydroxide must be heated. This procedure is expensive and requires excessive quantities of external heat.
Similarly, U.S. Pat. No. 4,147,650 to Sabatelli and Daugherty teaches a method for making an alkaline detergent slurry using sodium hydroxide and/or sodium metasilicate, sodium tripolyphosphate and water. The amount of caustic is strictly monitored. Amounts greater than 40 to 50 percent by weight sodium hydroxide are avoided as causing solidification to occur.
Thus, it would be desirable to provide a method for making a solid, cast alkaline detergent which incorporates sequestering agents homogeneously therein. It is also desirable that the process require temperatures below 144.degree. to 146.degree. F. It is also desirable that the process employ a sequestering agent having an effectiveness similar to sodium tripolyphosphate. It is desirable that the sequestrant employed have solubility properties compatible with those of the other detergent components.